Ultraviolet upconversion emission of Pb²⁺ ions sensitized by Yb³⁺-trimers in CaF₂

Under 978 nm near infrared (NIR) excitation, ultraviolet (UV) upconversion (UC) emissions from Pb²⁺ ions in CaF₂:Pb²⁺,Yb³⁺ were first observed at room temperature. The UC emission centered at ∼383 nm was ascribed to the ³P₀ → ¹A_1g (¹S₀) transition of Pb²⁺ ions. From transient measurements, the UC process was found to be dominated by the energy transfer process: three excited Yb³⁺ ions simultaneously transfer their energy to one Pb²⁺ ion. With the increase in Pb²⁺ concentration, the cooperative luminescence from 3-Yb³⁺ clusters decreased gradually until it was extinguished. The cooperative sensitization to one Pb²⁺ ion comes from three excited Yb³⁺ ions, which is a four-ion process and exhibits a third power dependence on the NIR pumping power.